CONTRIVANCE A NECESSITY. 141 



the end in view. The other half consists in this — 

 that the feathers of a bird's wing are made to 

 underlap each other, so that in the downward stroke 

 the pressure of the air closes them upwards against 

 each other, and converts the whole series of them 

 into one connected membrane, through which 

 there is no escape ; whilst in the upward stroke 

 the same pressure has precisely the reverse effect 

 — it opens the feathers, separates them from each 

 other, and converts each pair of feathers into a 

 self-acting valve, through which the air rushes at 

 every point. Thus the same implement is changed 

 in the fraction of a second from a close and con- 

 tinuous membrane which is impervious to the air, 

 into a series of disconnected joints through which 

 the air passes without the least resistance — the 

 machine being so adjusted that when pressure is 

 required the maximum of pressure is produced, 

 and when pressure is to be avoided, it is avoided 

 in spite of rapid and violent action. 



This, however, exhausts but a small part of the 

 means by which Law is made to do the work of 

 Will in the machinery of flight. It might easily be 

 that violent and rapid blows struck downwards 



